The present invention relates to the field of fluid and soil sampling methods and apparatus. Modern industries produce contaminants which have been released onto land. The contaminants migrate downward into the subsurface creating potential health risks. Contaminant remediation plans are implemented to remove soil and ground water contamination.
Designing a remediation plan typically requires collecting soil and fluid samples to determine the extent of subsurface contamination. The term fluid as used herein refers to both gas and liquid. Soil samples provide subsurface data including contaminant concentration for inorganic and organic compounds, grain size, mineral composition, texture, density, permeability and porosity. Fluid samples are analyzed to determine contaminant concentration, organic chemistry in the case of soil gas, and both organic and inorganic chemistry in the case of liquid.
A conventional method of collecting soil and soil gas samples is to drill a borehole to a desired sampling depth and lower a soil sampling device into the bottom of the borehole. Soil sampling devices typically have a hollow interior and are driven into the formation by repetitive percussion. As the device is driven into the formation a soil sample is forced into the hollow interior. The sampling device is removed from the borehole to retrieve the soil sample. A soil gas probe is then lowered into the borehole and driven into the formation to collect a gas sample.
A problem with the conventional method of collecting soil and soil gas samples is that during the time between retrieval of the soil sampling device and lowering of the soil gas probe, the gas in the subsurface immediately below the bottom of the borehole may be released into the borehole atmosphere before it can be collected by the soil gas probe. Off-gassing results from decreased lithostatic load due to removal of soil in the borehole. The off-gassing into the borehole will likely reduce the soil gas concentration readings.
A further problem with the known method is that the soil and soil gas samples are not collected from the same depth. When constructing a contaminant distribution model it is highly desirable to have both soil and fluid samples from the same depth for direct correlation between various soil and fluid data.
A second conventional method for extracting soil and gas samples from the same depth is to first drive the soil gas probe into the bottom of the borehole and collect a soil gas sample. The soil gas probe is then removed from the borehole and a soil sampling device is lowered into the borehole. The soil sampling device is driven around the hole produced by the soil gas probe. The soil sampler is then removed from the borehole to recover the sample. The soil sample will include a cylindrical depression formed by the soil gas probe.
A problem with the second conventional method of collecting soil and soil gas samples from the same depth is that the soil sample is manifestly disturbed by the collapsed hole made by the gas probe. The collapsed hole adversely affects various measurements, such as permeability, porosity and texture. The soil sample may also be chemically biased by off-gassing during soil gas sample collection. Off-gassing may affect, for example, the amount of volatile organics in the soil sample.
Conventional fluid and soil sampling devices collect either soil or fluid samples. Before each device is lowered into the borehole the device is decontaminated so that the sampling is not tainted. A problem with conventional fluid and soil sampling devices is that each device must be decontaminated, lowered into the borehole, and removed from the borehole to collect each individual sample. The increased operating time necessary to extract both soil and fluid samples increases the cost of extracting the samples.